pmctools: whole-system, low-overhead performance measurement in freebsd
DESCRIPTION
An overview of the PmcTools toolkit in FreeBSD, presented at ACM Bangalore. The presentation briefly touches upon: the motivations and goals of the project, the programming APIs, some aspects of the implementation and on possible future work.TRANSCRIPT
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PmcTools: Whole-system, low-overheadperformance measurement in FreeBSD
Joseph Koshy
FreeBSD Developer
18 April 2009
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Outline
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Introduction
Goals Of This Talk
Introduce FreeBSD/PmcTools.Introduce BSD culture.
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Introduction
About FreeBSD
http://www.freebsd.org/Popular among appliance makers, ISPs, web hosting providers:
Fast, stable, high-quality code, liberal license.
FreeBSD culture in one sentence: “Shut up and code”.
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Introduction
About [email protected]
FreeBSD developer since 1998.Technical interests:
Performance analysis; the design of high performance software.Low power computing.Higher order, typed languages.Writing clean, well-designed code.
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Introduction
The Three Big Questions in Performance Analysis
1 What is the system doing?2 Where in the code does the behaviour arise?3 What is to be done about it?
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Introduction
Question 1: What is the system doing?
System behaviour:Traditional UNIX tools: vmstat, iostat, top, systat, ktrace,truss.Counters under the sysctl hierarchy.Compile time options such as LOCK PROFILING.New tools like dtrace.
Machine behaviour:Modern CPUs have in-CPU hardware counters measuringhardware behaviour: bus utilization, cache operations, instructionsdecoded and executed, branch behaviour, floating point and vectoroperations, speculative execution, . . .Near zero overheads, good precision.
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Introduction
Question 2: Which portion of the system isresponsible?
Which subsystems are involved?Where specifically in the code is the problem arising?System performance is a “global” property.
“Local” inspection of code not always sufficient.As a community we are still exploring the domain of performanceanalysis tools:
Which data to collect.Collecting it with low-overheads.Making sense of the information collected.
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Introduction Introducing PmcTools
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Introduction Introducing PmcTools
PmcTools Project Goals
Have Fun!
Measurements
Low−overheads
FreeBSD Tier−1
Use in production
Architectures
Platform for
PmcTools
Whole−system
Interesting tools
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Introduction Introducing PmcTools
Performance Analysis: Conventional vs. PmcTools
Description Conventional PmcToolsNeed special binaries Yes NoDynamically loaded objects No YesProfiling scope Executable Process & SystemNeed restart Yes NoMeasurement overheads High LowProfiling tick Time Many optionsProfile inside critical sections No Yes (x86)Cross-architecture analysis No YesDistributed profiling No YesProduction use No Yes
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Introduction Introducing PmcTools
Related open-source projects
Linux Many projects related to PMCs:Oprofile: http://oprofile.sourceforge.net/Perfmon: http://perfmon2.sourceforge.net/Perfctr: http://sourceforge.net/projects/perfctr/Rabbit: http://www.scl.ameslab.gov/Projects/Rabbit/
Solaris CPC(3) library.NetBSD A pmc(3) API.
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PmcTools Architectural Overview
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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PmcTools Architectural Overview
Overview: Architecture
libpmc
kernel
pro
cess
log
pro
cess
pro
cess
hwpmc
CP
U
CP
U
CP
U
CP
U
CP
U
CP
U
A platform to build tools that use PMC data.
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PmcTools Architectural Overview
Components
hwpmc kernel bitskernel changes (see later)
libpmc application APIpmccontrol management tool
pmcstat proof-of-concept applicationpmcannotate contributed tool
etc... others in the future
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PmcTools Architectural Overview
PMC Scopes
#1
CPU 1 CPU 2 CPU 3
Process−scope PMC
System−scope PMC
#0
#2
#3
#4
CPU 0
1 process-scope PMC & 2 system-scope PMCs simultaneously active.
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PmcTools Architectural Overview
Counting vs. Sampling
System−scope,Process−scope,
Process−scope, System−scope,
Counting Counting
Sampling Sampling
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PmcTools Architectural Overview
Using system-scope PMCs
pro
cess
kernel
pro
cess
Three system scope PMCs, on three CPUs.Measure behaviour of the system as a whole.
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PmcTools Architectural Overview
Using process-scope PMCs
kernel
ow
ner
targ
et
2: attach
1:
allo
cate
A process-scope PMC is allocated & attached to a target process.Entire “row” of PMCs reserved across CPUs.
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PmcTools API
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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PmcTools API
API Overview
Categories:Administration (2).Convenience Functions (8).Initialization (1).Log file handling (3).PMC Management (10).Queries (7).Arch-specific functions (1).
32 functions documented in 15 manual pages. 10 manual pages forsupported hardware events.
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PmcTools API
Example: API Usage
read
start
attach
allocate
release
stop
pmc allocate() Allocate a PMC; returns a handle.pmc attach() Attach a PMC to a target process.pmc start() Start a PMC.pmc read() Read PMC values.pmc stop() Stop a PMC.pmc release() Release resources.
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PmcTools Design Issues
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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PmcTools Design Issues
PMCs Vary A Lot
AMD Athlon XP 4 PMCs, 48 bits wide.AMD Athlon64 4 PMCs, Different set of hardware events.Intel Pentium MMX 2 PMCs. 40 bits wide. Counting only.Intel Pentium Pro 2 PMCs, 40 bits for reads, 32 bits for writes.Intel Pentium IV 18 PMCs shared across logical CPUs. En-
tirely different programming model.Intel Core Number of PMCs and widths vary. Has
programmable & fixed-function PMCs.Intel Core/i7 As above, but also has per-package PMCs.
PMCs are closely tied to CPU micro-architecture.PMC capabilities, supported events, access methods,programming constraints can vary across CPU generations.
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PmcTools Design Issues
API Design Issues
Issues:
Designing an extensible programming interface for application use.Allowing knowledgeable applications to make full use of hardware.
PmcTools philosophy:
Make simple things easy to do.Make complex things possible.
Current “UI” uses name=value pairs:
% pmcstat -p k8-bu-fill-request-l2-miss,\mask=dc-fill+ic-fill,usr
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PmcTools Profiling
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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PmcTools Profiling
Conventional Statistical Profiling
Needs specially compiled binaries (cc -pg).Sampling runs off the clock tick.
Cannot profile inside kernel critical sections.
“In-place” record keeping.Call graph is approximated.
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PmcTools Profiling
PmcTools’ Statistical Profiling
Sets up PMCs to interrupt the CPU on overflow.Uses an NMI to drive sampling (on x86):
Can profile inside kernel critical sections.Needs lock-free implementation techniques.
Separates record keeping from data collection.Captures the exact callchain at the point of the sample.
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PmcTools Profiling
Profiling with NMIs
hwpmc helper
thread
lock−less ring
buffers
per−owner
log buffer
hardclock()
CPU 2 CPU 3
NMI
record sample
hwpmc_hook()
log file
trap()
#0
#1
#2
#3
CPU 0 CPU 1
machine dependent trap handler
HWPMC(4)
low level trap handler
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PmcTools Profiling
Profiling Workflow
System/application
pmcstat
hwpmc(4) logfile
gprof
gprof(1) data
under investigation
gprof(1) output
pmcstat
Uses gprof(1) to do user reports (currently):Needs to be redone: gprof(1) limitations.
Call chains are captured and used to generate call graphs.
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PmcTools Profiling
Profiling of Shared Objects
rtld
0 0xFFFFFFFF
text
arguments
text data bss stackheap Kernelmmap
unmapped
da
ta
da
ta
da
ta
text
command line
Each executable object in the system gets its own gmon.out file:kernelkernel modulesexecutablesshared librariesrun time loader
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PmcTools Profiling
Remote Profiling
log data
console
analysis
measurement
system under
pmc configure log() takes a file descriptor.Can log to a disk file, a pipe, or to a network socket.Events in log file carry timestamps for disambiguation.
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PmcTools Implementation
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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PmcTools Implementation
Implementation Information
Module Commentssys/dev/hwpmc,sys/sys/pmc*.h
31K LoC, i386&amd64
lib/libpmc 3.3K LoCusr.sbin/* 5.4K LoCdocumentation 29 manual pages, 11K LoD
All public APIs have manual pages.All hardware events, and their modifiers are documented.The internal API between libpmc and hwpmc is also documented.
See also: “Beautiful Code Exists, If You Know Where To Look”, CACM,July 2008.
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PmcTools Implementation
Impact on Base Kernel
Space Requirements 2 bits (P HWPMC, TDP CALLCHAIN). Uses freebits in existing flags words.
Kernel Changes Clock handling, kernel linker, MD code, processhandling, scheduler, VM (options HWPMC HOOKS).
Kernel Callbacks
hwpmc
scheduler
kernel linker
VM (mmap)
exec
low−level (assembler)
clock handling
trap()
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PmcTools Implementation
Portability
Application Portability High.Portability of libpmc Moderate. Requires a POSIX-like system.Adding support for new PMC hardware Moderate.Kernel bits Low.
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Status & Future Work Status
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Status & Future Work Status
Current State
Proof-of-concept application pmcstat is the current “userinterface”.
Crufty.
Low overheads (design goal: 5%) and tunable.In production use. Being shipped on customer boxes by appliancevendors.Support load on the rise (esp. requests for support of newhardware).
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Status & Future Work Status
Support load
Volunteer project. Initial hardware bought from pocket, or on loan.Current hardware support:
12 combinations of {PMC hardware × 32/64 bit OS variants} × 9OS versions [FreeBSD 6.0 · · · 6.4, 7.0 · · · 7.2, 8.0] = 108combinations!Need a hardware lab to manage testing and bug reports.
Need an automated test suite that is run continuously.Also useful for detecting OS & application performance regressionsearly.
Email support load is on the rise:Rise in FreeBSD adoption.FreeBSD users and developers worldwide now chipping in withfeatures, bug fixes, offering tutorials and spreading the word.
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Status & Future Work Future Projects
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Status & Future Work Future Projects
Profiling the Cloud
traffic
Control/Data
Analysis Console
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Status & Future Work Future Projects
Other project ideas
A graphical visualizer “console”.Enhance gprof, or write a report generator afresh.Link up with existing profile based optimization frameworks.Allow performance analysis of non-native architectures.Support non-x86 PMCs.Integrate PmcTools and DTrace.Port to other BSDs and/or OpenSolaris.
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Status & Future Work Research Topics
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Status & Future Work Research Topics
Profile guided system layout
L1/L2/L3
Cache
ker
nel
applications
Lay out the whole system to help “hot” portions remain in cache.Would require an augmented toolchain(http://elftoolchain.sourceforge.net/) & enhancements tohwpmc(4).Useful for low end devices using direct-mapped caches.
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Status & Future Work Research Topics
Detection of SMP data structure layout bugs
struct shared...char sh foo;int sh bar;char sh buzz;...
;
Would use a combination of static analysis & hwpmc(4) data.Detection of the poor cache line layout behaviour.
Cache line ping-ponging between CPUs.
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Status & Future Work Research Topics
Profiling for power use
What part of the system consumes power?Where in the code is power being spent?
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Conclusion
1 IntroductionIntroducing PmcTools
2 PmcToolsArchitectural OverviewAPIDesign IssuesProfilingImplementation
3 Status & Future WorkStatusFuture ProjectsResearch Topics
4 Conclusion
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Conclusion
Talk Summary
FreeBSD/PmcTools was introduced.The design & implementation of PmcTools was looked at.Possible future development and research directions for theproject were touched upon.
Joseph Koshy (FreeBSD Developer) FreeBSD/PmcTools 18 April 2009 48 / 48